Documentation

This simulator provides hemispheric reflectance (albedo) of snow for unique combinations of impurity content (black carbon, dust, and volcanic ash), snow grain size, and incident solar flux characteristics. The simulator is a single-layer implementation of the Snow, Ice, and Aerosol Radiation (SNICAR) model (Flanner et al., 2007; Flanner et al., 2009), which utilizes the two-stream radiative transfer solution of Toon et al. (1989).

Input fields

Incident radiation: Radiative flux incident on the top of snowpack can be either direct-beam or diffuse (equal intensity from all upward directions).
Solar zenith angle: If the incident flux is direct, users must specify the sun angle (relative to the zenith direction).
Surface spectral distribution: The spectral distribution of incident radiation is unique for different atmospheric states, including cloudiness and gaseous composition. Four characteristic atmosphere options are currently provided. Surface spectral distribution can have a large bearing on the snow broadband albedo (averaged over the solar spectrum), as can be seen by comparing results under "clear-sky" and "cloudy" atmospheres, even though spectral albedo profiles appear similar. Clouds strongly absorb near-infrared radiation, leaving a higher proportion of surface flux as visible energy, which snowpack reflects strongly, thus causing higher broadband albedo. We derived the spectral profiles using an atmospheric radiative transfer model (Zender et al., 1997) with "standard" AFGL atmospheric profiles.
Snow grain effective radius: This is the surface area-weighted mean radius of the collection of ice particles composing the snowpack. This property has strong influence on near-infrared reflectance and on the magnitude of albedo reduction caused by absorbing impurities. Resolution of input is 1 μm.
Snowpack thickness and density: The product of these two terms defines snowpack mass (units of kg/m2), which is input to the radiative transfer model. Combinations of thickness and density that have an identical product produce identical albedo.
Albedo of underlying ground: Users specify reflectance of the surface beneath snowpack for visible and near-infrared broadband intervals. We assume the prescribed values are constant across each spectral interval. Underlying ground albedo only influences reflectance of relatively thin snowpack, but density and effective grain size also determine the influence of underlying ground.

Impurities and notes about optical properties:

Users can specify mass concentrations of two types of black carbon, four sizes of dust particles, and one type of volcanic ash. We derive the optical properties for these absorbing impurities and ice grains with Mie solutions (Bohren and Huffman, 1983), using various indices of refraction and assumptions of particle size distribution. The two types of black carbon are: 1) uncoated (mimicking hydrophobic particles), with properties tuned to achieve a mass absorption cross-section of 7.5 m2/g at 550 nm (Bond and Bergstrom, 2006); and 2) sulfate-coated black carbon (mimicking hydrophilic black carbon), which is composed of a weakly-absorbing shell (sulfate) surrounding black carbon, resulting in an absorption enhancement (per unit mass of black carbon) of about 1.5 (Bond et al., 2006). We also obtain optical solutions for the core/shell configuration with Bohren and Huffman's solver. Properties for dust are unique for each of four size bins, which represent partitions of a lognormal size distribution. Dust optical properties depend strongly on source material and these properties are designed to represent "global-mean" characteristics as closely as possible. We apply the Maxwell-Garnett approximation for combining indices of refractions, assuming a mixture of quartz, limestone, montmorillonite, illite, and hematite. Some dust particles (e.g., those containing a large proportion of strongly-absorbing hematite) can have a larger impact on snow albedo than the dust applied here (e.g., Aoki et al., 2006; Painter et al., 2007). Finally, volcanic ash also exhibits large optical variability, depending on volcanic mineralogy and type of eruption. The properties we apply here are derived from measurements of ash particles from the Mount St. Helens eruption (Patterson, 1981; Steve Warren, personal communication).

Radiative transfer:

The radiative transfer solution applied here is a single-layer version of the multiple scattering, multi-layer approximation described by Toon et al. (1989), with the delta-hemispheric mean approximation. We simulate fluxes within 470 spectral bands at 10 nm resolution from 0.3–5.0 μm. Early single-layer snow albedo solutions and research on the influence of absorbing impurities include Wiscombe and Warren (1980) and Warren and Wiscombe (1980). Dozier and Painter (2004) review the influence of snowpack properties on multi-angular and multi-spectral radiative transfer and remote sensing.

Acknowledgment:

Data obtained from this simulator may be freely used in publications or presentations. When doing so, please include a reference to: "SNICAR-Online (Flanner et al., 2007)."

References (cited above):

Aoki, T., H. Motoyoshi, Y. Kodama, T. J. Yasunari, K. Sugiura, and H. Kobayashi (2006), Atmospheric aerosol deposition on snow surfaces and its effect on albedo, SOLA, 2, 13-16, doi: 10.2151/sola.2006-004.

Bohren, C. F., and D. R. Huffman (1983), Absorption and Scattering of Light by Small Particles, 530 pp., John Wiley & Sons, New York, NY.

Bond, T. C., and R. W. Bergstrom (2006), Light absorption by carbonaceous particles: An investigative review, Aerosol Sci. Technol., 40 (1), 27-67, doi: 10.1080/02786820500421521.

Bond, T. C., G. Habib, and R. W. Bergstrom (2006), Limitations in the enhancement of visible light absorption due to mixing state, J. Geophys. Res., 111, D20211, doi: 10.1029/2006JD00731.

Dozier, J. and T. H. Painter (2004), Multispectral and hyperspectral remote sensing of alpine snow properties, Annu. Rev. Earth Planet. Sci., 32,465-494, doi: 10.1146/annurev.earth.32.101802.120404.

Flanner, M. G., C. S. Zender, J. T. Randerson, and P. J. Rasch (2007), Present day climate forcing and response from black carbon in snow, J. Geophys. Res., 112, D11202, doi:10.1029/2006JD008003.

Flanner, M. G., C. S. Zender, P. G. Hess, N. M. Mahowald, T. H. Painter, V. Ramanathan, and P. J. Rasch (2009), Springtime warming and reduced snow cover from carbonaceous particles, Atmos. Chem. Phys., 9, 2481-2497.

Painter, T. H., A. P. Barrett, C. C. Landry, J. C. Neff, M. P. Cassidy, C. R. Lawrence, K. E. McBride, and G. L. Farmer (2007), Impact of disturbed desert soils on duration of mountain snow cover, Geophys. Res. Lett., 34, L12502, doi:10.1029/ 2007GL030284.

Patterson, E. M. (1981), Measurements of the imaginary part of the refractive index between 300 and 700 nanometers for Mount St. Helens ash, Science, 211, 836-838.

Painter, T. H., A. P. Barrett, C. C. Landry, J. C. Neff, M. P. Cassidy, C. R. Lawrence, K. E. McBride, and G. L. Farmer (2007), Impact of disturbed desert soils on duration of mountain snow cover, Geophys. Res. Lett., 34, L12502, doi:10.1029/ 2007GL030284.

Toon, O. B., C. P. McKay, T. P. Ackerman, and K. Santhanam (1989), Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres, J. Geophys. Res., 94 (D13), 16,287-16,301.

Warren, S., and W. Wiscombe (1980), A model for the spectral albedo of snow. II: Snow containing atmospheric aerosols, J. Atmos. Sci., 37, 2734-2745.

Wiscombe, W. J., and S. G. Warren (1980), A model for the spectral albedo of snow. I: Pure snow, J. Atmos. Sci., 37, 2712-2733.

Zender, C. S., Brett Bush, Shelly K. Pope, Anthony Bucholtz, William D. Collins, Jeffrey T. Kiehl, Francisco P. J. Valero, and John Vitko, Jr. (1997), Atmospheric absorption during the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE), J. Geophys. Res., 102(D25), 29901-29915.


Comments and questions may be addressed to: Mark Flanner